Lynette Feeney

THE ULTRASTRUCTURE OF THE RETINAL VESSELS. II. THE SMALL VESSELS.

Adult human retinal capillaries differ from those in other organs in having thicker basement membranes, closer spaced pericytes, and extensive “Swiss cheese” cavitation of the outer wall at the junction of the basement membranes of the pericytes and glial cells. Dense inclusions are observed in some of these cavitations. Capillary basement membranes in the rat and monkey retina are thinner, and cavitation is very slight in the monkey and absent in the rat. No dense inclusions were seen in the cavitations of the monkey capillary. The possibility exists that the cavitations are a result of aging or an artifact. However, they may represent a potential perivascular space which could be of importance in the formation of the diabetic aneurysm and in perivascular exudation. Arterioles 8 μ in diameter are surrounded by a single smooth muscle cell but otherwise are identical in structure to the capillary, including cavitation of the basement membranes. In slightly larger arterioles, the external basement membrane system contains collagen fibers which increase in number as the vessel enlarges. Elastic tissue is not demonstrable. The capillary wall is continuous with venules and small veins, with little change in structure except for increasing amounts of collagen to form an adventitia. Cavitation also is found in the outer basement membrane system of venules and smaller veins. The latter are distinguished from capillaries also by their size.

THE ULTRASTRUCTURE OF THE RETINAL BLOOD VESSELS. I. THE LARGE VESSELS.

The large veins and arteries in the normal human, monkey, and rat retina have been studied by electron microscopy. Vessels in the four retinal quadrants extending from the optic disc to the periphery were studied. The arterial system is especially endowed with smooth muscle. Arteries near the disc have a luminal diameter of 100 μ and possess five to seven layers of muscle in their walls. At the equator there are three to five muscle layers, and in the peripheral retina, one to two. In contrast to other muscular arteries, no elastin could be demonstrated on the wall of these vessels. Also, no internal elastic lamina was found, even at the disc edge. Thick, often laminated, basement membranes were observed between layers of smooth muscle cells. The walls of the veins contained neither elastic tissue nor smooth muscle. The media is composed of widely spaced pericytes whose cytoplasm differ from that of smooth muscle in having fewer filaments and less dense attachment zones along the plasma membrane. An adventitia of loosely arranged collagen constitutes two-thirds of the thickness of the wall. All the vessel were entirely ensheathed by glial cell cytoplasm, and no perivascular space was observed between the adventitia and the glial cells. Nerve endings were not found in or on the walls of these vessels. The peculiar anatomic arrangement of the structures in the walls of the arteries may account for the responsiveness of the arteries to pressor substances, and their reaction in diseases such as hypertension and eclampsia. The media of the large arteries in the human retina often contained dense, feathery, osmiophilic material, whereas it was absent in the arteries of animals.

THE ULTRASTRUCTURE OF THE RETINAL VESSELS. III. VASCULAR-GLIAL RELATIONSHIPS.

Three types of cells form the perivascular glia: Muller cells, and two types of astrocytes. The Muller cytoplasm surrounds the major portion of the capillary walls in all layers, with occasional contributions from thn accessory glia. The large arteries and veins are ensheathed by glial cytoplasm derived equally from Muller cells and accessory glia. Through out its length, the Muller cell cytoplasm contains glycogenlike particles, numerous fibers, and smooth-surfaced vesicles. The perivascular Muller cell basement membranes merge with the adventitial ground substance of the vessel walls, without forming a perivascular space. Vessels which lack an adventitia, such as small arterioles, venules, and capillaries, show a fusion of the Muller cell basement membrane with that of adjacent smooth muscle cells, pericytes, or endothelial cells. The two astrocytic glial cells are as follows: a. A fibrous branching type, chiefly found on the walls of large arteries. The cytoplasm contains few organelles and a mat of fine fibrils in the perinuclear region. Some cytoplasmic extensions contain many fibrils whereas others contain few. b. A type with few branchings, a diffusely fibrillar cytoplasm, considerable rough-surfaced endoplasmic reticulum, and many mitochondria, chiefly found near vein and capillary walls.

Alcian Blue and Colloidal Iron Staining Methods Adapted to Filter Paper Electrophoresis

Filter paper moistened with solutions used in electro-chromatography was spotted with 0.5-1.0μl of solutions of mucopolysaccharides and allowed to air dry. Substances tested with respect to their staining reaction were as follows: (a) From commercial sources: hyaluronic acid, heparin, chondroitin sulfate, ovomucoid and gastric mucin, (b) From natural sources: blood serum, saliva, tears, vitreous filtrate and aqueous humor. Alcian blue was found to be a good general stain for mucopolysaccharides and for locating such material on filter paper, especially when more specific means were used subsequently for identifying the kind of mucopolysaccharide present. Staining by colloidal iron of materials on filter paper was similar to that by the periodic acid-Schiff reaction. However, heparin and chondroitin sulfate were not stained by iron when on filter paper but were stained when placed on glass slides.